Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (LTE). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-Carrier Frequency Division Multiple Access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in Rel. 8 LTE.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2.
The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs.
The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Layer 1/Layer 2 (L1/L2) Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other data related information (e.g. HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a sub-frame, assuming that the user allocation can change from sub-frame to sub-frame. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, the L1/2 control signaling needs only be transmitted once per TTI. The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). It should be noted that in 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH.
With respect to scheduling grants, the information sent on the L1/L2 control signaling may be separated into the following two categories, Shared Control Information (SCI) carrying Cat 1 information and Downlink Control Information (DCI) carrying Cat 2/3 information.
Shared Control Information (SCI) Carrying Cat 1 Information
The shared control information part of the L1/L2 control signaling contains information related to the resource allocation (indication). The shared control information typically contains the following information:                A user identity indicating the user(s) that is/are allocated the resources.        RB allocation information for indicating the resources (Resource Blocks (RBs)) on which a user(s) is/are allocated. The number of allocated resource blocks can be dynamic.        The duration of assignment (optional), if an assignment over multiple sub-frames (or TTIs) is possible.        
Depending on the setup of other channels and the setup of the Downlink Control Information (DCI)—see below—the shared control information may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Downlink Control Information (DCI) Carrying Cat 2/3 Information
The downlink control information part of the L1/L2 control signaling contains information related to the transmission format (Cat 2 information) of the data transmitted to a scheduled user indicated by the Cat 1 information. Moreover, in case of using (Hybrid) ARQ as a retransmission protocol, the Cat 2 information carries HARQ (Cat 3) information. The downlink control information needs only to be decoded by the user scheduled according to Cat 1. The downlink control information typically contains information on:                Cat 2 information: Modulation scheme, transport-block (payload) size or coding rate, MIMO (Multiple Input Multiple Output)-related information, etc. Either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated resource blocks)        Cat 3 information: HARQ related information, e.g. hybrid ARQ process number, redundancy version, retransmission sequence number        
Downlink control information occurs in several formats that differ in overall size and also in the information contained in its fields. The different DCI formats that are currently defined for LTE Release 8/9 (3GPP LTE) are described in detail in 3GPP TS 36.212, “Multiplexing and channel coding (Release 9)”, version 8.8.0 or 9.0.0, section 5.3.3.1 (available at www.3gpp.org and incorporated herein by reference).
Downlink & Uplink Data Transmission
Regarding downlink data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH), along with the downlink packet data transmission. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the data is transmitted (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the UE (receiver) to identify the resources on which the data is transmitted.        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling, this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier (“cross-carrier scheduling”). This other, cross-scheduled component carrier could be for example a PDCCH-less component carrier, i.e. the cross-scheduled component carrier does not carry any L1/L2 control signaling.        The Transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g. the number of resource blocks assigned to the user equipment)) allows the user equipment (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. The modulation scheme may be signaled explicitly.        Hybrid ARQ (HARQ) information:                    HARQ process number: Allows the user equipment to identify the hybrid ARQ process on which the data is mapped.            Sequence number or new data indicator (NDI): Allows the user equipment to identify if the transmission is a new packet or a retransmitted packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a PDU prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation).            UE Identity (UE ID): Tells for which user equipment the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.                        
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the user equipment about the transmission details. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the user equipment should transmit the data (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling, this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier.        
This other, cross-scheduled component carrier may be for example a PDCCH-less component carrier, i.e. the cross-scheduled component carrier does not carry any L1/L2 control signaling.                L1/L2 control signaling for uplink grants is sent on the DL component carrier that is linked with the uplink component carrier or on one of the several DL component carriers, if several DL component carriers link to the same UL component carrier.        The Transport Format, the user equipment should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g. the number of resource blocks assigned to the user equipment)) allows the user equipment (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.        Hybrid ARQ information:                    HARQ Process number Tells the user equipment from which hybrid ARQ process it should pick the data.            Sequence number or new data indicator: Tells the user equipment to transmit a new packet or to retransmit a packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a protocol data unit (PDU) prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).            UE Identity (UE ID): Tells which user equipment should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.                        
There are several different possibilities how to exactly transmit the information pieces mentioned above in uplink and downlink data transmission. Moreover, in uplink and downlink, the L1/L2 control information may also contain additional information or may omit some of the information. For example:                HARQ process number may not be needed, i.e. is not signaled, in case of a synchronous HARQ protocol.        A redundancy and/or constellation version may not be needed, and thus not signaled, if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre-defined.        Power control information may be additionally included in the control signaling.        MIMO related control information, such as e.g. pre-coding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.        
For uplink resource assignments (on the Physical Uplink Shared Channel (PUSCH)) signaled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore, it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e. the RV info is embedded in the transport format (TF) field. The Transport Format (TF) respectively modulation and coding scheme (MCS) field has for example a size of 5 bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating redundancy versions (RVs) 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RV0. The size of the CRC field of the PDCCH is 16 bits.
For downlink assignments (PDSCH) signaled on PDCCH in LTE the Redundancy Version (RV) is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signaled on PDCCH. 3 of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries modulation order and Transport block size info are signaled.
Physical Downlink Control Channel (PDCCH)
The physical downlink control channel (PDCCH) carries the L1/L2 control signaling, i.e. transmit power control commands and the scheduling grants for allocating resources for downlink or uplink data transmission. To be more precise, the downlink control channel information (i.e. the DCI contents, respectively, the L1/L2 control signaling information) is mapped to its corresponding physical channel, the PDCCH. This “mapping” includes the determination of a CRC attachment for the downlink control channel information, which is a CRC calculated on the downlink control channel information being masked with an RNTI, as will explained below in more detail. The downlink control channel information and its CRC attachment are then transmitted on the PDCCH (see 3GPP TS 36.212, sections 4.2 and 5.3.3).
To form the PDCCH payload, the DCI undergoes channel coding: addition of a CRC attachment followed by convolutional coding and rate matching according to PDCCH format capacity. The coded DCI bits i.e. PDCCH payload, are then mapped to Control Channel Elements (CCEs) according to the PDCCH format. The UE finds the PDCCH specific to it by monitoring a set of PDCCH candidates (a set of consecutive CCEs on which PDCCH could be mapped) in every subframe. The UE uses its Radio Network Temporary Identifier (RNTI) to try and decode candidates. The RNTI is used to de-mask a PDCCH candidate's CRC. If no CRC error is detected the UE determines that PDCCH carries its own control information. Each scheduling grant is defined based on Control Channel Elements (CCEs). Each CCE corresponds to a set of Resource Elements (REs). In 3GPP LTE, one CCE consists of 9 Resource Element Groups (REGs), where one REG consists of four REs.
The PDCCH is transmitted on the first one to three OFDM symbols within a sub-frame. For a downlink grant on the physical downlink shared channel (PDSCH), the PDCCH assigns a PDSCH resource for (user) data within the same sub-frame. The PDCCH control channel region within a sub-frame consists of a set of CCE where the total number of CCEs in the control region of sub-frame is distributed throughout time and frequency control resource. Multiple CCEs can be combined to effectively reduce the coding rate of the control channel. CCEs are combined in a predetermined manner using a tree structure to achieve different coding rate.
In 3GPP LTE (Release 8/9), a PDCCH can aggregate 1, 2, 4 or 8 CCEs. The number of CCEs available for control channel assignment is a function of several factors, including carrier bandwidth, number of transmit antennas, number of OFDM symbols used for control and the CCE size, etc. Multiple PDCCHs can be transmitted in a sub-frame.
Downlink control channel information in form of DCI transports downlink or uplink scheduling information, requests for aperiodic CQI reports, or uplink power control commands for one RNTI (Radio Network Terminal Identifier). The RNTI is a unique identifier commonly used in 3GPP systems like 3GPP LTE (Release 8/9) for destining data or information to a specific user equipment. The RNTI is implicitly included in the PDCCH by masking a CRC calculated on the DCI with the RNTI—the result of this operation is the CRC attachment mentioned above. On the user equipment side, if decoding of the payload size of data is successful, the user equipment detects the DCI to be destined to the user equipment by checking whether the CRC on the decoded payload data using the “unmasked” CRC (i.e. after removing the masking using the RNTI) is successful. The masking of the CRC code is for example performed by scrambling the CRC with the RNTI.
In 3GPP LTE (Release 8) the following different DCI formats are defined:                Uplink DCI Formats:                    Format 0 used for scheduling of PUSCH            Format 3 is used for transmission of TPC commands for PUCCH and PUSCH with 2 bit power adjustments (multiple UEs are addressed)            Format 3A is used for transmission of TPC commands for PUCCH and PUSCH with single bit power adjustments (multiple UEs are addressed)                        Downlink DCI Formats:                    Format 1 used for transmission of DL SCH assignments for SIMO operation, i.e. scheduling of one PDSCH codeword            Format 1A used for compact transmission of DL SCH assignments for SIMO operation, i.e. used for compact scheduling of one PDSCH codeword and random access procedure initiated by a PDCCH order            Format 1B used to support closed loop single rank transmission with possibly contiguous resource allocation, i.e. for compact scheduling of one PDSCH codeword with precoding information            Format 1C is for downlink transmission of paging, RACH response and dynamic BCCH scheduling, i.e. is used for very compact scheduling of one PDSCH codeword            Format 1D is used for compact scheduling of one PDSCH codeword with precoding and power offset information            Format 2 is used for transmission of DL-SCH assignments for closed-loop MIMO operation            Format 2A is used for transmission of DL-SCH assignments for open-loop MIMO operation                        
For further information on the LTE physical channel structure in downlink and the PDSCH and PDCCH format, see Stefania Sesia et al., “LTE—The UMTS Long Term Evolution”, Wiley & Sons Ltd., ISBN 978-0-47069716-0, April 2009, sections 6 and 9.
Component Carrier Structure in LTE
The downlink component carrier of a 3GPP LTE system is subdivided in the time-frequency domain in so-called sub-frames. In 3GPP LTE each sub-frame is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each sub-frame consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each of OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consist of a number of modulation symbols transmitted on respective NRBDL×NscRE subcarriers as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g. employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 4. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.9.0 or 9.0.0, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
The term “component carrier” refers to a combination of several resource blocks. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Further Advancements for LTE (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved in the 3GPP. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In Carrier Aggregation (CA), two or more Component Carriers (CCs) are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several carriers in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz, even though these carriers in LTE are in different frequency bands. On the aggregated wider channel, an LTE-Advanced user equipment can access several spectrum fragments simultaneously. Meanwhile, an LTE user equipment can access only one spectrum fragment of them, thus meeting the need for spectral compatibility as well as reducing the costs of bits.
All component carriers can be configured to be 3GPP LTE Release 8/9 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. This does not necessarily mean that all component carriers need to be compatible to 3GPP LTE (Release 8/9).
A user equipment may simultaneously receive or transmit on one or multiple component carriers. On how many component carriers simultaneous reception/transmission is possible, is depending on the capabilities of a user equipment.
A 3GPP LTE (Release 8/9) compatible user equipment can receive and transmit on a single CC only, provided that the structure of the CC follows the 3GPP LTE (Release 8/9) specifications, while a 3GPP LTE-A (Release 10) compatible user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the 3GPP LTE (Release 8/9) numerology.
It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may not be possible to configure a UE with more uplink component carriers than downlink component carriers.
In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not to provide the same coverage.
The spacing between centre frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively.
When carrier aggregation is configured, the user equipment only has one RRC connection with the network. One cell—the special cell—provides the security input and the NAS mobility information (e.g. TAI). There is only one special cell per UE in connected mode.
At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g. TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected mode. Within the configured set of component carriers, other cells are referred to as Secondary Cells (SCells). The characteristics of the downlink and uplink PCell are:                The uplink PCell is used for transmission of Layer 1 uplink control information        The downlink PCell cannot be de-activated, unlike SCells        Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF        The downlink PCell cell can change with handover        Non-access stratum information is taken from the downlink PCell        PCell can only be changed with handover procedure (i.e. with security key change and RACH procedure)        PCell is used for transmission of PUCCH        
The configuration and reconfiguration of component carriers can be performed by RRC. Activation and deactivation is done via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. When adding a new SCell, dedicated RRC signalling is used for sending the system information of the SCell, the information being necessary for transmission/reception (similarly as in Rel-8/9 for handover).
When a user equipment is configured with carrier aggregation there is one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as ‘DL anchor carrier’. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats, called CIF.
A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carriers does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
FIGS. 7 and 8 exemplarily show possible linkages between downlink and uplink component carriers. While in FIG. 7 all downlink component carriers are linked to the same uplink component carrier, in FIG. 8 downlink component carriers 1 and 2 are linked to uplink component carrier 1 and downlink component carrier 3 is linked to uplink component carrier 2.
LTE-A Support of Relaying Functionality
Relaying is considered for LTE-Advanced (Rel. 10 architecture) as a tool to improve e.g. the coverage of high data rates, group mobility, temporary network deployment, the cell-edge throughput and/or to provide coverage in new areas.
The relay node may be wirelessly connected to a radio-access network via a donor cell. The connection can be                inband, in which case the network-to-relay link share the same band with direct network-to-user equipment links within the donor cell. Rel. 8 user equipments should be able to connect to the donor cell in this case.        outband, in which case the network-to-relay link does not operate in the same band as direct network-to-user equipment links within the donor cell.        
With respect to the knowledge in the user equipment, relays can be classified into                transparent, in which case the user equipment is not aware of whether or not it communicates with the network via the relay.        non-transparent, in which case the user equipment is aware of whether or not it is communicating with the network via the relay.        
Depending on the relaying strategy, a relay may be part of the donor cell or may control cells of its own.
In the case the relay is part of the donor cell, the relay does not have a cell identity of its own (but may still have a relay ID. In this case, a relay should preferably support also LTE Rel. 8 user equipments. Smart repeaters, decode-and-forward relays and different types of L2 relays are examples of this type of relaying.
In the case the relay is in control of cells of its own, the relay controls one or several cells and a unique physical-layer cell identity is provided in each of the cells controlled by the relay. From a user equipment perspective there is no difference in accessing cells controlled by a relay and cells controlled by a “normal” eNodeB. The cells controlled by the relay should support also LTE Rel. 8 user equipments. Self-backhauling (L3 relay) uses this type of relaying.
In FIG. 9 and FIG. 10 an exemplary LTE-A system is shown which utilizes relay nodes (RN). The wireless interface between eNode B and RN, which connects a RN with the radio access network, is referred to as S1 interface.
Relay Backhaul Subframes
If the eNB-to-relay link operates in the same frequency spectrum as the relay-to-UE link, simultaneous eNB-to-relay and relay-to-UE transmissions on the same frequency resource may not be feasible unless sufficient isolation of the outgoing and incoming signals is provided due to the relay transmitter causing interference to its own receiver.
Therefore, when the relay node transmits data to the DeNB, it cannot receive data from UEs attached to the relay (r-UEs). Likewise, when the relay node receives data from the DeNB, it cannot transmit data to the r-UEs on the same frequency as shown in FIG. 10.
Thus, there is subframe partitioning between the relay backhaul link (eNB-to-relay link) and relay access link (relay-to-UE link). Currently it has been agreed that:                Relay backhaul DL subframes, during which DeNB-to-relay downlink backhaul transmissions may occur, are semi-statically assigned.        Relay backhaul UL subframes, during which relay-to-DeNB uplink backhaul transmissions may occur, are implicitly derived by HARQ timing from relay backhaul DL subframes.        
In relay backhaul DL subframes, the relay node can receive from or transmit to (if it is scheduled 4 ms earlier) the DeNB, so r-UEs are not supposed to expect any relay transmission. In order to support backward compatibility for r-UEs, the relay node configures backhaul DL subframes as MBSFN subframe in relay.
MBSFN subframe can be configured for every 40 ms, so relay backhaul DL subframes also support 40 ms configuration. And the same as MBSFN subframe configuration, relay backhaul DL subframes of the SCell cannot be configured at subframes #0, #4, #5 and #9. Those subframes that are not allowed to be configured as backhaul DL subframes are called “illegal DL subframes” in the following.
Time-frequency resources shall be set aside for eNB-RN transmissions by time multiplexing eNB-RN and RN-UE transmissions. Subframes, during which eNB-RN transmissions may take place, are configured by higher layers. Downlink subframes configured for eNB-to-RN transmissions shall be configured as MBSFN subframes by the relay node. eNB-to-RN transmissions occur in downlink subframes and RN-to-eNB transmissions occur in uplink subframes. For frame structure type 1, eNB-to-RN and RN-to-UE transmissions occur in the downlink frequency band, while RN-to-eNB and UE-to-RN transmissions occur in the uplink frequency band.
For frame structure type 1, a subframe configured for eNB-to-RN transmission is a subframe satisfying [(10·nf+└[n/2┘])mod 8]εΔBSC with the exception that a downlink subframe that cannot be configured as MBSFN subframe in the relay node cell shall not be configured for eNB-to-RN transmission. nf is the system frame number and ns is the slot number within a radio frame. The set ΔBSC is determined as the union of the applicable offset values listed in the following Table with respect to the parameter SubframeConfigurationFDD, which is configured by higher layers, and where “x” means that the corresponding bit in the bitmap can be either 0 or 1.
SubframeConfigurationFDDOffset value element of ΔBSC{xxxxxxx1}7{xxxxxx1x}6{xxxxx1xx}5{xxxx1xxx}4{xxx1xxxx}3{xx1xxxxx}2{x1xxxxxx}1{1xxxxxxx}0
The above MBFSN configuration is standardized in TS 36.215, Chapter 5.2 (available at www.3gpp.org and incorporated herein by reference).
There are various MBSFN subframe configurations, according to which either 3, 6, 9, 12, 18, 21 or 24 subframes out of 40 subframes are configured for DL on the relay backhaul DL link. For instance, 3-out-of-40 subframes can be used by the DeNB to transmit data on the downlink over the backhaul link to the relay node. One particular configuration of the 3-out-of-40 configuration is to use the subframes #3, #11 and #27. Other subframes may be used as well, e.g. #2, #18 and #26. In this case, a roundtrip time of 8 ms (=8 subframes) is assumed, and therefore when starting at subframe #2, subframes #10 and #34 are not possible since these are illegal DL subframes, as explained above.
It is important to note that an inband relay, even if it cannot transmit and receive on the same frequency on the same subframe, it still can transmit control signalling to the UEs on the access link on the same frequency, on a subframe in which it receives a DL transmission. This is possible due to the gap created by the switching of the receiver to transmitter mode on the half-duplex relays. This gap is usually 2˜3 OFDM symbols length and these are used to send control signalling to the UEs in the access link.
Though the following explanations for describing the underlying problem of the invention and for describing the various embodiments of the invention only relates to the 3-out-of-40 subframe configuration, the present invention is not restricted to this particular subframe configuration. Rather, any of the above-mentioned subframe configurations for MBSFN can be used, and the skilled person would easily apply the principles of the invention to the different subframe configurations.
Basic Scenario
FIG. 11 discloses a network scenario in which three relay nodes and two UEs are directly connected to a DeNB. Furthermore, several UEs are connected to relay node RN1. Before carrier aggregation, a serving cell is configured in the backhaul and one in the access link of the communication system. Initially, the relay nodes are operating in an outband mode, i.e. the carrier frequency of the serving cell used on the backhaul link (f2) is different from the carrier frequency of the serving cell used on the access link (f1). Correspondingly, the resources on the backhaul link (between DeNB and RNs) to RN1 are shared between the three relay nodes and the two UEs that are directly connected to the DeNB. The capacity of the resources for the UN link (also called backhaul link in the following) corresponds to the resources allocated to the RN1 out of the whole resources of the frequency band f2. On the other hand, the resources of the Uu link (between the relay node and the UEs, also called access link) are only shared between the UEs directly connected to RN1.
The access link has to handle much more control information than the backhaul link, since there is a one-to-many mapping on the downlink and a many-to-one mapping on the uplink for data and control information with respect to the relay node.
A significant increase of traffic on the Uu access link (f1) may put the backhaul link (f2) under stress. In particular, it may be assumed that the capacity of the backhaul link (f2) is 10 Mbps for RN1. The total capacity of the access link (f1) from RN1 is 20 Mbps. However, due to the backhaul link (f2) for RN1 being limited to 10 Mbps, traffic may be served only with 10 Mbps over the access link (f1) from RN1 to the attached UEs.
When the traffic coming down to the UEs attached to the RN1 increases to a critical point, the DeNB may decide to use carrier aggregation over the backhaul link. If the DeNB decides to use carrier aggregation due to the high amount of traffic, a secondary carrier over the backhaul link will be created (SCell). Assuming the SCell over the backhaul link has a capacity of 5 Mbps, the traffic which can be served after carrier aggregation increases from 10 Mbps (without carrier aggregation) to 15 Mbps (out of a total possible capacity of 20 Mbps).
This is depicted in FIG. 12, showing the SCell used over the backhaul link to the relay nodes, RN1, RN2 and RN3.
If the aggregated SCell uses a frequency that is the same as the carrier frequency used over the access link (f1), the operation of the relay node will be inband with regard to the SCell, while outband for the PCell. Correspondingly, the SCell will not affect the operation of the initial carrier (f2) over the backhaul link, which will then be used as the Primary carrier (PCell) in the carrier aggregation mode. In addition, UEs under the relay node 1 must continue to use the same frequency f1, in order to not force a handover.
It may become a quite common scenario for carrier aggregation that the SCell will have to use the same carrier frequency as the one used over the access link. The reason is that each operator can only use a certain limited number of frequency bands, and when aggregating a new carrier, it may be necessary to select the same carrier frequency for the SCell in the backhaul link as in the access link over the access link.
FIG. 13 depicts the subframes for the DeNB, RN and r-UE, and in the lower part the HARQ process IDs used by the r-UE to receive data in the particular subframe number. The DeNB and the RN are also configured for HARQ, wherein however the numbering of the HARQ process IDs is omitted in said respect. It can be assumed that the RN in communication with the r-UE uses the same HARQ processes, wherein communications between the relay node and the DeNB may or may not use different HARQ processes than over the access link (RN-r-UE). For instance, the relay node will also use HARQ process ID #4 for subframes 4, 12, 20 etc., the same as the r-UE.
The subframes are depicted for the PCell (f2) and the SCell (f1) after using carrier aggregation as explained above in connection with FIG. 12. As can be seen from FIG. 13, subframes 0, 4, 5, 9 of every radio frame are illegal DL subframes for the DL transmission over the backhaul link. In other words, though these subframes 0, 4, 5 and 9 may be used for UL in the backhaul (i.e. to receive data from the relay node) they cannot be used by the DeNB to transmit data to the relay node.
Based on the 3-out-of-40 subframe configuration explained above, the DeNB transmits the PDCCH to the relay node at subframe #3. The PDCCH for subframe #3 is assumed to include downlink data, but no UL grant for the relay node to transmit data back to the DeNB on the PUSCH (PDCCH is transmitted, but omitted from FIG. 13, subframe #3). With respect to the data, the relay node will transmit an ACK/NACK to the DeNB on the PUCCH in the PCell.
In subframe #11 the relay node receives a uplink grant from the DeNB, and thus a PUSCH transmission will be performed by the relay node in subframe #15 (#11+4 subframes by implicit configuration). Since the relay node uses subframe #15 for transmitting data to the DeNB on the SCell (f1), the relay node cannot receive data from the r-UEs on said carrier frequency f1. The subframe #15 is blocked for uplink transmissions from the r-UEs attached to said relay node.
Since the relay node does not receive an UL grant in subframe #3, the relay node will not perform an uplink transmission of data on the PUSCH over the Un interface in subframe #7. Therefore, subframe #7 will not be blocked for the SCell, and theoretically the subframe #7 could be used by the relay node to receive an uplink transmission from the r-UE.
Since the relay node receives data from the DeNB in subframe #3, it is not possible for the relay node to transmit data over the PDSCH to the r-UE in the same subframe of the SCell, because the relay node cannot switch from receiver to transmitter in the same subframe.
It is important to note that a relay node having an inband SCell, even if it cannot transmit and receive data (PDSCH, PUSCH) on the same frequency on the same subframe (as shown in FIG. 13, subframe #3), it still can transmit control signalling (i.e. PDCCH) on the same frequency on a subframe in which it receives a DL transmission. This is possible due to the gap created by the switching of the receiver to transmitter mode on the half-duplex relays. This gap usually has a length of 2-3 OFDM symbols, and these may be used to send control signalling to the UEs in the access link, including e.g. an UL grant for the r-UE.
Therefore, though the relay node cannot transmit data to the r-UE in subframe #3, it would still be possible to schedule the r-UE to perform an uplink transmission on subframe #7, using the first 3 OFDM symbols of subframe #3, as explained above. Furthermore, as explained above, subframe #7 would not be blocked on the SCell, since no PUSCH transmission is scheduled for the relay node, due to the missing UL grant in subframe #3.
However, transmitting an UL-grant from the relay node to the r-UE in subframe #3 is also not possible because the relay node needs to first decode the PDCCH it received from the DeNB on the backhaul link to determine whether the corresponding subframe #7 (#3+4 subframes) will be free or not. The relay node starts decoding subframe #3, which however takes longer than one subframe (1 ms). Hence, the relay node will miss the opportunity to schedule the UEs on the access link on the same subframe #3 for UL data on the subframe #7, even though subframe #7 in this scenario would not be blocked by a PUSCH transmission from the relay node to the DeNB.
In the downlink subframe #11, the DeNB transmits data on the PDSCH and an UL grant on the PDCCH. Upon receiving, the relay node starts decoding subframe #11 , and thus will schedule a PUSCH transmission in subframe #15 (#11+4 subframes). Subframe #15 is only an illegal subframe for downlink; therefore, the DeNB may receive the PUSCH from the relay node in subframe #15.
Again, a downlink transmission from the relay node to the r-UE is not possible for subframe #11, since the relay node receives data in said subframe #13. Further, an UL grant for the UE would also not be transmitted from the relay node to the r-UE, because the subframe #15 is blocked to receive data from the r-UE of carrier f1 and because decoding of subframe #11 takes too long for the UL-grant to be transmitted to the r-UE in subframe #11.
In summary, in the present scenario in which the relay node operates inband for the SCell, typical inband problems arise, as just explained. For instance, not all subframes are available to send/receive on the access link due to subframe blocking, which limits the downlink and uplink data rates. Furthermore, even if the subframes are available, the relay node is not able to use them to schedule the r-UEs in the access link due to the time it takes the relay node to decode the UL grants in the DL assignments to determine in the first place if the subframe is available or not.